1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) backlight unit employing a plurality of optical devices, and more particularly, to a backlight unit improved in light mixing feature, in which light beams outputted from the optical devices are effectively dispersed and mixed together, thereby producing uniform white light free from color blur.
2. Description of the Related Art
A Cold Cathode Fluorescent Lamp (CCFL) has been adopted for a light source of a conventional Liquid Crystal Display (LCD) backlight. The CCFL, which uses a mercury gas, may trigger environmental pollution. Besides, the CCFL has low response rate, low color reproducibility, and inappropriateness for a lighter and smaller LCD panel.
In contrast, a Light Emitting Diode (LED) is environment-friendly, quick to respond with a rate of several nano seconds and effective in video signal stream. Moreover, the LED can be driven impulsively, and can reproduce color by 100% or more and adjust a light amount of red, green and blue LEDs to alter luminance and color temperature arbitrarily. Also, the LED is beneficial for a lighter and smaller LCD panel, thereby actively employed as a backlight source of e.g., the LCD panel.
As described above, the CCFL or LCD backlight directly irradiates light toward the LCD panel from a surface light source which is substantially identical in size with the LCD panel.
As shown in FIG. 1, the conventional LCD backlight 200 includes a surface light source 220 and an optical sheet 230. The surface light source 220 has a plurality of optical devices 210 such as green, blue and red LEDs arranged thereon. The optical sheet 230 is disposed over the surface light source 220 to output light beams incident from the surface light source 220 to the outside.
In the conventional LCD backlight 200, the optical sheet 230 disposed over the surface light source 220 includes a diffuser sheet 236 and at least one light collecting sheet 238. The diffuser sheet 236 diffuses the light beams incident from the optical devices 210 uniformly. The light collecting sheet 238 collects the light beams diffused by the diffuser sheet 236 in a direction perpendicular to the plane of the LCD panel.
Furthermore, the conventional LCD backlight unit 200 may further include a protective sheet 240 disposed above the light collecting sheet 238 to protect an underlying optical structure.
The surface light source 220 includes a reflecting layer 256 formed on a substrate 250 to reflect the light beams emitted from the optical devices 210 upward.
Therefore, as shown in FIG. 2, in order to combine the red, green and blue lights into white light, the conventional LCD backlight 200 adopts the surface light source 220 having the optical devices 210 arranged thereon and the optical sheet 230 fixedly disposed over the surface light source 220.
In the conventional LCD backlight 200, when the light beams emitted from the optical devices 210 of the surface optical source 220 are outputted to the outside through the optical sheet 230, significant difference in the amount of output light is observed between some areas where the optical devices 210 are located and the other areas where the optical devices are not disposed. This resultantly causes color stains.
That is, to gain product competitiveness, the conventional LCD backlight 200 should be competitive in terms of not only costs but also quality. One factor concerns whether to output uniform white light without color blur.
Therefore, to acquire product competitiveness in terms of costs, the conventional LCD backlight 200 should be preferably equipped with a fewer number of the optical devices 210 per unit area, which accordingly widens a pitch D between the optical devices 210.
With the increase in the pitch between the optical devices 210 as described above, the light amount outputted to the areas between the optical devices 210 as shown in FIG. 2 is significantly smaller than that outputted to the areas just over the optical devices 210. Accordingly, the surface light source 220 of the conventional LCD backlight fails to output uniform white light due to color stains.
That is, in the conventional LCD backlight 200, a majority of the light beams outputted from the optical devices 210 directly transmit the optical sheet 230 to be output to outside.
The optical sheet 230 disperses the light beams from the optical devices 210 and mixes them together, however, with negligible effects.
As a result, as shown in a graph of FIG. 2, in the conventional LCD backlight 200, there exists a great difference in the light amount outputted to the areas just over P1 the optical devices 210 and that outputted to the areas between P2 the optical devices, thereby not obtaining uniform white light owing to color blur.